PARENT SESSION
Posters P7B Evolution of photosynthesis. Abstracts (579-591)

Evolution of photosynthesis: Implications from the structural comparison of Photosystem I and II. Ingo Grotjohann^*,, Craig Jolley¹, Petra Fromme¹, ¹ Department of Chemistry and Biochemistry, Tempe, Arizona, USA

ABSTRACT- Both photosystems in oxidative photosynthesis, PSI and PSII, have evolved from a common ancestor. The most recent structural data on both, PSI (Jordan et al., 2001) and PSII (Ferreira et al., 2004) allow a more thorough comparison. Despite low sequence homologies between the central subunits of PSI (PsaA and PsaB) and PSII (PsbA to PsbD), the overall arrangement of the transmembrane helices is similar, especially in the antenna regions. Large differences in length and tilt can be found between the transmembrane helices that coordinate most carriers of the ETC. A comparison of both ETC's shows that the principal structural arrangement has been conserved, although the angle of the primary donor pair differs vastly between both photosystems. A large difference can be found in the electrostatic environment: whereas P700 is embedded in a hydrophobic environment, P680 is surrounded by 10 polar amino acids in less than 1.2 nm distance. This may be one of the factors responsible for the difference in the redox potential of PSI and PSII. The stromal loops of the core show a comparably high degree of similarity, with PSI possessing a longer loop for coordination of the Fx cluster. The lumenal loops of PSI serve for the coordination of chlorophyll and for the docking of plastocyanine or cytochrome c6, whereas the bulky loops in PSII build a closed "basket"-like structure around P700 with the oxygen evolving complex at the side. Whereas the core regions of PSI coordinate a total of 25 chlorophylls, in PSII these are only 2. The reason why PSII never developed a similar central antenna like PSI may be the necessary facilitation of the frequent exchange of PsbC after photodamage. Interestingly, the antenna moieties show 20 counterparts regarding the coordinated chlorophylls; this suggests that the ancient antenna protein predecessor might have contained about 10 chlorophylls.

KEY WORDS: photosystem I, evolution, photosystem II, structure